In a conventional autofocus apparatus that performs a focusing operation using the high-frequency components of a video signal, a prescribed frequency component extracted by a band-pass filter is extracted from a video signal in a signal processing circuit and lens control is carried out in such a manner that the prescribed frequency component will take on as large a value as possible in order to prevent against the effects of noise. This utilizes a phenomenon in which, if the focused state has been achieved, image sharpness increases and high-frequency components increase in relative terms. This type of autofocus apparatus merely involves providing a frequency-component extracting circuit within a video signal processing circuit and thus it frequently finds use in inexpensive consumer-oriented video cameras.
The prescribed frequency component necessary to implement the above-described autofocus operation is a prescribed spatial frequency component in the spatial frequency region of video displayed on a screen. The spatial sampling frequency, which is important when extracting the prescribed spatial frequency component, changes depending upon the number of pixels in the video signal.
In a conventional video camera that merely records moving pictures, the number of pixels in an image signal from output by a CCD image sensor to recording on video tape is 768 horizontally×525 vertically or 720 horizontally×480 vertically owing to various format limitations involved in recording a moving picture on video tape. The former number of pixels is in accordance with the NTCS video format and the latter in accordance with the DVC recording format. In an autofocus apparatus, therefore, a fully satisfactory autofocus operation can be obtained if use is made of a filter for extracting a spatial frequency component, which is optimum in terms of performing the autofocus operation, from a video signal of the number of pixels mentioned above.
Recent consumer-oriented video cameras not only record moving images on video tape but also are equipped with a so-called digital camera function that enables still images to be captured and recorded in an image memory. In such recording of still images, there is no limitation on format relating to number of pixels in the images and therefore the number of pixels used in still images is steadily increasing in order to achieve higher image quality for such images. For example, in a situation where a still image composed of a total of 1,300,000 pixels has been captured, the image sensor provides a 1280H×960V video signal.
When a filter that extracts the optimum frequency component for autofocus from a 768H×525V video signal is used to extract the aforesaid frequency component from a 1280H×960V video signal, the number of pixels is approximately 1.5 times as large and therefore the spatial sampling frequency also is about 1.5 times as large. This means that the frequency component for autofocus also becomes approximately 1.5 times that of the optimum component. The problem that arises is that the desired autofocus operation can no longer be carried out. Specifically, even though photography is performed in one and the same video camera, the accuracy of autofocus at the time of moving-picture photography differs from that of still-picture photography.
Further, since an image sensor has a large number of pixels, as mentioned above, it has been contemplated to execute electronic wide-screen processing (zoom processing) by reducing the multipixel image. In this case also, however, the image used in presenting the screen display still has an elevated spatial sampling frequency because the image originally has a large number of pixels. Consequently, the optimum spatial frequency component used in autofocus can no longer be obtained and the desired autofocus operation can no longer be carried out just as before.